Adsorptive separations using carbon molecular sieves as adsorbents are well known in the prior art for resolving various gas mixtures, including air. Such separations are predicated upon the compositions of the gas mixtures and the components' selectivity for adsorption on adsorbents, such as carbon molecular sieves.
The use of nitrogen in industrial gas applications has seen significant growth, particularly with the development of non-cryogenic gas mixture separation. A major field of nitrogen separation comprises the separation of nitrogen from air. The removal of nitrogen from air results in an enriched oxygen gas component which is more strongly adsorbed by appropriate carbon molecular sieves which are selective for oxygen adsorption. When nitrogen is desired as a product, typically at elevated pressure, it is desirable to adsorb oxygen from air to result in unadsorbed nitrogen enriched product passing over an oxygen selective carbon molecular sieve adsorbent. The oxygen is then removed during a stage of desorption, typically at lower pressure. This results in nitrogen being recovered at the pressure of the feed air, while oxygen is recovered at a pressure below the feed air pressure. As a result, for the production of nitrogen without significant pressure loss in an adsorptive separation of air, it is desirable to utilize oxygen selective carbon molecular sieve adsorbents for the separation.
In the context of adsorptive air separations using carbon molecular sieves, processes were first described in U.S. Pat. No. 3,801,513. U.S. Pat. No. 4,415,340 discloses an air separation process using carbon molecular sieves in which the initial product from an adsorptive bed just going onstream is recycled to the feed air, upstream of the pump or compressor providing the feed air to the separatory equipment.
Oxygen spiking in nitrogen gas from an air separation process, which was addressed in the above prior art patent, was further addressed in U.S. Pat. No. 4,256,469 which utilizes a relatively slow final repressurization to preclude oxygen spiking when a regenerated adsorption bed goes back on line.
This oxygen spiking was also addressed in U.S. Pat. No. 4,264,339 which provides a continually increasing pressure in an adsorption bed going through the adsorption step in an adsorptive air separation.
U.S. Pat. No. 4,925,461 discloses an adsorptive air separation using carbon molecular sieves wherein after pressure equalization between two adsorption beds, product nitrogen is allowed to backflow into the adsorption bed about to go back on the adsorption step in order to avoid oxygen spiking.
U.S. Pat. No. 4,717,407 in one of its several embodiments discloses a membrane preceding another form of gas separation, such as adsorption. However, the membrane permeates the undesired component for further resolution in the adsorption unit, while the reject is a first desired component stream.
U.S. Pat. No. 4,765,804 discloses that a depressurization gas from an adsorptive separation can be contacted with a membrane with the permeate desired component recycled as feed to the adsorptive separation.
Other patents of general interest to adsorptive separation include U.S. Pat. Nos. 3,738,087; 4,348,213 and 4,853,004.
The prior art has typically relied on slow repressurizations, pressure equalization and depressurization gas permeate recycle steps to provide energy efficiency and high recoveries in adsorptive air separations. Also, high purities in nitrogen product have been attempted by slow repressurizations, product-to-product end equalization of adsorption beds and recycle of initial produced nitrogen product having oxygen spiking. However, the prior art has not resolved the problems of attaining high efficiency, high recovery and high nitrogen purity while avoiding the adsorbent material fluidization and dusting resulting from energy saving pressure equalization. These problems have been overcome by the present invention which is set forth in greater detail below.